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Question #44155

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The Question

Consider the fully conjugated monocyclic polyenes: Benzene (I), Cyclooctatetraene (II), and the di-trans isomer of [10]annulene (III). Let $x$ , $y$ , and $z$ represent the number of distinct carbon-carbon bond lengths present in the most stable ground-state conformation of I, II, and III, respectively. Which of the following correctly identifies $x$ , $y$ , and $z$ , and provides the correct rationale for the electronic nature of III?

Options

A.
$x=1$ , $y=2$ , $z=2$ ; III is non-aromatic due to transannular H-H steric repulsion forcing a non-planar geometry, leading to localized single and double bonds.
B.
$x=1$ , $y=2$ , $z=1$ ; III is aromatic because it possesses $10\pi$ electrons, satisfying Hückel's $(4n+2)$ rule, resulting in a uniform bond order of $1.5$ .
C.
$x=1$ , $y=1$ , $z=1$ ; I, II, and III are all planar aromatic systems due to extended conjugation, resulting in complete delocalization.
D.
$x=1$ , $y=2$ , $z=2$ ; III is anti-aromatic because the non-planar distortion forces the $10\pi$ system to adopt a Möbius topology with a phase inversion.

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Exception_Exposure

Learn to identify edge cases and boundary exceptions that JEE examiners hide in math problems.

Concepts Behind This Question

Bond Order in Resonance Hybrids|Aromaticity (Huckel’s Rule)

Why This Question Is Tricky

Common Mistakes

The core trap tests the boundary conditions of Hückel's Rule.

Students often rote-memorize that $10\pi$ electrons ( $4n+2$ ) automatically confers aromaticity, leading them to incorrectly assume a uniform bond order of 1.5 for [10]annulene.

The 'Hidden Variable' is the geometric constraint.

In the di-trans isomer of [10]annulene, transannular H-H steric repulsion makes a planar conformation physically impossible.

Because planarity is a strict prerequisite for the continuous p-orbital overlap required for aromaticity, the molecule puckers.

This non-planarity breaks the resonance hybrid, reverting the system to a simple cyclic polyene with alternating, localized single and double bonds.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Chemical Bonding and Molecular Structure - Class 11 Part 1
Concept	Bond Order in Resonance Hybrids
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free